This proposal seeks to define angiogenic responses to Wnt secreted growth factors (Wnts) and to probe their roles in vascular development and physiology. Wnts act through the frizzled receptors to activate several distinct signaling cascades including the Wnt/beta-catenin, Wnt/Ca2+ , and Wnt/planar cell polarity (PCP) pathways, the latter two referred to as "non-canonical" pathways. The overall objective is to determine the relative roles of the Wnt/beta-catenin and Wnt/Ca2+ pathways in endothelial and mural cells. Our hypothesis is that Wnt/Frizzled signaling is critical for proper vasculogenesis, angiogenesis, and vascular function. Our general strategy will be to define cellular and physiological consequences of selectively altering Wnt/beta-catenin and Wnt/Ca2+. signaling in vascular cells using in vitro and in vivo approaches. Our preliminary studies suggest that both the canonical and non-canonical pathways can stimulate endothelial cell growth and survival. In addition, mutation of frizzled genes in mice and man implicate the Wnt/beta-catenin and Wnt/Ca2+. pathways in physiological angiogenesis. The Wnt/beta-catenin pathway may also affect angiogenesis indirectly, as this branch of the pathway induces angiogenic genes, such as Vascular Endothelial Growth Factor (VEGF) and Interleukin-8 (IL-8). We will determine whether Wnts and a new Frizzled ligand, Norrin, can act as angiogenic factors by stimulating cultured endothelial cells to grow, survive, and participate in angiogenic morphogenesis. The Wnt/Frizzled cascades that are activated in endothelial and vascular smooth muscle cells in response to Wnts, Norrin, and Frizzleds will be defined. We hypothesize that Wnt/beta-catenin may promote angiogenesis both directly and indirectly, by inducing VEGF or IL-8 in mural cells that, in turn, stimulate endothelial cells. Wnt/beta-catenin function in vascular development will be studied using mouse models that conditionally activate or inactivate signaling in either endothelial or mural cells. A negative regulator of Wnt/beta-catenin signaling, Axin, will be used to interfere with Wnt/beta-catenin signaling in vivo. To activate Wnt/beta-catenin signaling, we will express a stabilized form of beta-catenin specifically and conditionally in the vasculature. Embryonic and adult viability will be assessed and vascular morphogenesis, angiogenesis and vascular function will be analyzed. These studies will define how aberrant Wnt/Frizzled signaling might contribute to vascular pathologies and diseases, such as familial exuditive vitreoretinopathy.